reducerComposition.js

function postComments(state = [], action) {
  switch(action.type) {
    case 'LOAD_COMMENTS' :
      return action.comments;
    case 'ADD_COMMENT':
      return [...state, {
        user : { username: action.author },
        text : action.comment      
      }];
    case 'REMOVE_COMMENT':
      return [
        ...state.slice(0, action.i),
        ...state.slice(action.i + 1)
      ];
    default:
      return state;
  }
}

function comments(state = {}, action) {
  if (typeof action.postId !== 'undefined') {
    return {
      ...state,
      [action.postId]: postComments(state[action.postId], action)
    };
  }

  return state;
}